A conventional thyratron, in its simplest form as schematically illustrated in FIG. 1, comprises a flat anode plate 1 facing a cathode 2 with a control grid 3 located between them, this electrode arrangement being contained within a gas filled envelope 4. When it is wished to trigger the thyratron into conduction, a positive potential may be applied to the control grid 3, which previously was held at a negative bias potential, and a discharge is developed between the anode 1 and cathode 2. The amount of current which the thyratron is capable of conducting satisfactorily is determined to some extent by the area of the anode and cathode facing surfaces. With conventional geometry, change of area is obtained by change of diameter and thus, if a larger current capability is required, the thyratron must be designed to have greater anode and cathode diameters.